1. Field of the Invention
The present invention relates to a band-pass elastic wave filter device, and more particularly to an elastic wave filter device including a low acoustic velocity film, a piezoelectric film, and an IDT electrode that are stacked on a high acoustic velocity member.
2. Description of the Related Art
WO12/086639 discloses a surface acoustic wave (SAW) filter formed by stacking, on a support substrate, an AlN film, a SiO2 film, a LiTaO3 film, and an IDT electrode predominantly made of Al. The SiO2 film, which is a low acoustic velocity film, is stacked between the AlN film, which is a high acoustic velocity film, and the LiTaO3 film, which is a piezoelectric film. This enables achieving higher frequencies and increases the Q value.
In contrast, WO12/114593 discloses a duplexer using a SAW filter. In a transmission filter of the duplexer, a comb capacitive electrode is connected as a bridging capacitance to a SAW resonator.
WO06/123518 discloses a boundary acoustic wave filter device. In the boundary acoustic wave device, a comb capacitive electrode is connected to a boundary acoustic wave filter. In this case, where Vm is the acoustic velocity of a boundary acoustic wave and Vc is the acoustic velocity of a slower transversal wave, out of the acoustic velocities of slow transversal waves in first and second media, in a direction orthogonal to a direction in which the electrode fingers of the comb capacitive electrode extend, Vm/λm<Vc/λc. Note that λm is the wavelength of a boundary acoustic wave, and λc is the wavelength of the above-mentioned slower transversal wave.
If a comb capacitive electrode such as that described in WO12/114593 is provided in the SAW filter with the multilayer structure described in WO12/086639, it has been discovered that undesired spurious responses occur in the passband of the SAW filter.
In WO06/123518, the influence of boundary acoustic waves generated by the comb electrode is merely suppressed in the boundary acoustic wave device, but is not suppressed in a SAW device. It is also necessary to increase Vc/λc. Thus, the pitch between the electrode fingers of the comb electrode becomes very small. Therefore, it is very difficult to manufacture the comb capacitive electrode.